For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes.
As the altitude of an aircraft increases, the ambient pressure outside of the aircraft decreases and, unless otherwise controlled, excessive amounts of air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin pressure altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin pressure altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin pressure altitude to minimize passenger discomfort.
In addition to controlling cabin pressure for passenger and crew comfort, many aircraft cabin pressure control systems also function to limit cabin differential pressure below one or more predetermined magnitudes. Cabin differential pressure refers to the pressure difference between the interior and exterior of the aircraft fuselage, and for many aircraft includes both a “positive” differential pressure limit and a “negative” differential pressure limit. A positive differential pressure occurs when the pressure within the fuselage is greater than the exterior pressure, and a negative differential pressure occurs when the pressure within the fuselage is less than the exterior pressure.
Regulations promulgated by various governmental certification authorities for many aircraft state that the aircraft needs to be equipped with systems and components that not only maintain aircraft cabin altitude within a relatively comfortable range, but that additionally limit cabin differential pressure below the aircraft's positive and negative limits. In addition, these same regulations state that at least two components be provided to limit cabin differential pressure below the positive limit, and at least two components be provided to limit cabin differential pressure below the negative limit.
To provide the functionality and redundancy stated in the above-noted regulations, aircraft cabin pressure control systems may be equipped with an outflow valve, one or more positive pressure relief valves, and one or more negative pressure relief valves, depending on system design and configuration. For example, in some aircraft, the cabin pressure control system includes an outflow valve, two positive pressure relief valves, and two negative pressure relief valves. This system configuration results in a total of five individual valve components and up to five separate penetrations in the aircraft fuselage. Other aircraft include an outflow control valve, which also provides a positive pressure relief function, a dual-function positive and negative pressure relief valve, and a negative pressure relief valve. This system configuration reduces the total number of valve components to three, but still results in three separate fuselage penetrations. Still other aircraft include an outflow valve, and two dual-function positive and negative pressure relief valves. Again, this system configuration results in three valve components and three fuselage penetrations.
Although the above-described aircraft cabin pressure control system configurations are robustly designed and are safe and reliable, each suffers certain drawbacks. For example, each valve component within the system increases the overall system and aircraft weight and can, in some cases, result in increased cost, complexity, and weight. Each valve component also takes up a certain amount of scarce interior space for both mounting and connection of and electrical and/or pneumatic interface. In addition, to the extent that each valve includes an individual fuselage penetration, there is a potential fuselage leakage source, and the accompanying maintenance associated with each valve. One or more of these factors can adversely affect aircraft initial and lifetime costs, as well as aircraft down time.
Hence, there is a need for an aircraft cabin pressure control system valve that has one or more of the following benefits: reduced overall system and aircraft weight; reduced number of fuselage penetrations; a reduced system space envelope; and, simplified system maintenance, without adversely affecting aircraft lifecycle costs. The present invention addresses one or more of these needs.